This invention relates to improved methods and apparatus for determining concentrations of interstitial oxygen in crystalline silicon by infrared (IR) spectroscopy.
Generally, oxygen atoms and other impurities contaminate crystalline silicon. As an example, crystals grown by the Czochralski process may have an oxygen concentration as high as 30 parts per million atomic (ppma) (ASTM F 1188-93). in the Czochralski process, oxygen from quartz crucibles used to hold a melt from which silicon crystals are grown will dissolve into the silicon melt. Also, oxygen in the ambient atmosphere surrounding the melt can likewise dissolve in the silicon melt. The oxygen dissolved in the melt is then incorporated into the crystal at the liquid-solid interface. Oxygen atoms typically occupy interstitial sites in the silicon unit cell and form two strong Si--O bonds with the nearest-neighbor silicon atoms.
The electrical performance of integrated circuit (IC) devices depends in part on the material properties of the silicon used in fabrication of the finished IC devices. The presence of oxygen in the silicon lattice is either beneficial or detrimental to device performance depending on its concentration. Therefore, improved silicon device manufacturing requires accurate measurement and control of the amount of dissolved oxygen in silicon crystals.
The semiconductor industry widely uses infrared (IR) spectroscopy for characterizing silicon based on the content of interstitial oxygen in crystals. Generally, silicon is transparent to IR radiation and the IR transmission or absorption spectrum exhibits several absorption bands caused by the vibrations of impurities in the silicon lattice. The standard method for measuring the interstitial oxygen content of silicon by Fourier transform infrared (FTIR) spectroscopy is described in the American Society for Testing and Materials (ASTM) Designation F 1188-93 entitled "Standard Test Method for Interstitial Atomic Oxygen Content of Silicon by Infrared Absorption". However, this method is limited to relatively thin samples of the crystal and require relatively expensive, inefficient and time-consuming steps for preparing the samples.